Re: Hierachical forums do not seem to work.
Aha, more info about the post-to-subforum error. Details are in Trac at https://trac.bbpress.org/ticket/674 but it appears that Kakumei may be scribbling on a global $forum_id variable deep in the bowels of bb-templates/kakumei/forum.php.
Fel64, when you couldn’t repeat the problem, were you using Kakumei?
I’m such a bbPress newb that I don’t know whether Kakumei falls outside the purview of bugs we should be reporting through Trac. Suggestions welcome.